In an actuator for moving a positioning element back and forth, particularly an electromagnetic actuator, the movement of the actuator armature is typically identical to the movement of the positioning element to be actuated, such that it is possible to measure the armature movement and consequently the movement of the positioning element in the region of the actuator.
In an electromagnetic actuator with two spaced-apart electromagnets, the pole faces of which point toward one another and between which an armature is guided such that it can be moved back and forth against the force of return springs when the electromagnets are alternately supplied with a current, a measurement of the current and/or voltage at the respective attracting magnet and during release of the restraining magnet makes it possible to draw conclusions about the armature movement that can subsequently be used for control purposes if the signals are processed appropriately.
An electromagnetic actuator of this type is used, for example, as a fully variable valve drive for actuating a gas exchange valve of a reciprocating internal combustion engine. In view of stricter requirements regarding control accuracy, particularly with respect to influencing the impact velocity of the armature on the pole face of the respective attracting magnet and therefore also the touch-down speed of the gas exchange valve on the valve seat, a measurement of movements derived from the current and voltage curves at the coils of the electromagnets no longer appears sufficient because the signals obtained therefrom can only be used for the subsequent engine cycle.
Consequently, it is necessary to measure the movement of the armature and therefore the movement of the positioning element “online” over the entire length of stroke with the aid of a corresponding sensor assembly, such that the power supply of the electromagnets can be influenced by appropriately controlling the actuator, for example, an electromagnetic actuator, based on corresponding signals while the actuator moves the positioning element. This makes it possible to control armature movement in the current engine cycle.
This requirement can only be fulfilled with a displacement sensor with a low error deviation that generates a corresponding signal during the entire stroke, i.e., a sensor that “reproduces” the stroke. Due to the requirements with respect to resolution and accuracy of gas exchange valves, as well as of injection nozzles and needle valves, associated with the relatively short strokes, the sensor assembly needs to be largely protected from interference. This also applies to other instances in which the movement of a reciprocating component, for example, a piston valve or the like, needs to be measured in a highly accurate fashion. In this case, the displacement signal being generated should be as linear as possible.
A sensor of this type is known, in principle, from DE 101 57 119 A, wherein this sensor requires, however, a relatively long structural length if accurate measurement signals are to be obtained.